TGF-beta dynamically regulates the differentiation of smooth muscle cells, and has been postulated to maintain vessel wall structure. TGF-beta also appears to possess immunosuppressive properties which protect the vascular endothelium against local inflammation and damage. Moreover, TGF-beta may inhibit the proliferation and migration of smooth muscle cells after vascular injury.
TGF-beta is synthesized as a latent peptide (FIG. 1). Latent TGF-beta refers to any of several complexes that include the 25 kD TGF-beta dimer in association with the latency associated peptide (LAP) or any of several additional TGF-beta binding proteins (LTBPs). Latent TGF-beta has no biological activity, i.e., it does not bind to the TGF-beta receptors.
The 25 kD TGF-beta dimer is also found associated with matrix components or other plasma proteins (FIG. 1). TGF-beta that is associated with matrix components or other plasma proteins is termed mature TGF-beta. This association also prevents the binding of TGF-beta to the TGF-beta receptors.
In addition to latent and mature forms of TGF-beta, which cannot bind to the TGF-beta receptors and which possess no known biological activity, TGF-beta also exists in forms which are capable of binding to the TGF-beta receptors and which elicit biological effects (FIG. 1). These forms of TGF-beta are termed "active TGF-beta." One example of a form of active TGF-beta is the 25 kD TGF-beta dimer which is free from association with LAP/LTBPs, or matrix or plasma components. The process(es) by which the latent form of TGF-beta is converted to the active form is termed "activation." The process(es) by which the mature form of TGF-beta is converted to the active form is termed "release."
Decreased levels of TGF-beta have been implicated in the development of atherosclerosis. Atherosclerosis is a disease of the major arteries, typified by changes in the vessel wall architecture. At lesion-prone sites where the endothelium becomes damaged or dysfunctional, smooth muscle cells from the media of the vessel migrate into the intima. At these sites, leukocytes, and in particular, monocytes and macrophages invade the expanded intima. As the lesion develops, lipid from the circulation is deposited into the intima (reviewed in Ross, Nature, 362, 801 (1993); Grainger et al. Biol. Rev. Camb. Philos. Soc., 70, 571 (1995)).
Agents which elevate TGF-beta activity, such as tamoxifen (TMX) (Grainger et al., Biochem. J., 294 109 (1993)) and aspirin (Grainger et al., Nat. Med., 1, 74 (1995)), can exhibit cardioprotective effects. However, the positive cardioprotective effects of these agents may be counterindicated by their potential side effects. TMX can cause liver carcinogenicity in rats, has been correlated with an increased risk of endometrial cancer in women and may increase the risk of certain gut cancers. Aspirin may result in ulcerogenesis and increased bleeding.
Agents which elevate TGF-beta levels may also be useful to prevent or treat diseases or conditions including cancer, Marfan's syndrome, Parkinson's disease, fibrosis, Alzheimer's disease, senile dementia, osteoporosis, diseases associated with inflammation, such as rheumatoid arthritis, multiple sclerosis and lupus erythematosus, and other auto-immune disorders. Such agents may also be useful to promote wound healing and to lower serum cholesterol levels.
Thus, there is a need for improved therapeutic methods and agents useful to maintain or elevate TGF-beta levels in mammals.